Use eslint instead of jshint

Add exclusions for all the currently-failing eslint rules.

Note that this also adds ecmaVersion = 6 for eslint, because
of a couple of for-of loops in PF_jsGrid.js, which could
probably be refactored to return to 5 if need be.

Bug: T210365
Change-Id: I6017303f2a668dac97f3c1d20a6851c04ea5ea7b